An embodiment of the present invention relates to a method and apparatus for determining acquisition geometry of an imaging system, as a function of parameters of the system, for an arbitrary position. An embodiment of the invention is more particularly directed to a method and apparatus for medical imaging. An embodiment of the invention is directed to providing greater precision and robustness to a geometry-determining operation of this kind. An embodiment of the invention can be applied to but not exclusively in the field of radiology and in particular to X-ray systems implementing methods of tomography or tomodensitometry.
There are known radiology systems that comprise a source of radiation, such as an X-ray tube, and a detector of the radiation, such as an X-ray detector, a table or support and a C-type arm, such as C-shaped arm or C-arm that is generally a vascular C-arm. These systems are capable of shifting in all three dimensions of a space. This mobility enables the practitioner to acquire images for any part whatsoever of the body of an object such as a patient lying on the table. In general, the table is capable of shifting in the three possible motions of translation associated with a given space, while the C-type arm is capable of shifting in the three possible rotations associated with this space. The practitioner uses a system of interventional radiology to have available tools such as a catheter or a coil within the patient's body, especially in the head. The geometry of the acquired images must be known with precision in order to help the practitioner in the positioning of the tools.
Two types of image acquisition are possible with these systems. The practitioner may acquire 2D images obtained by the projection of X-rays on the detector. Each image is obtained for a given position of the tube and of the detector. The part of the patient's body is then projected on the detector, in a conical projection. To guide these tools during a procedure, the practitioner may thus use these 2D images obtained with or without any contrast agent. The images acquired with low doses of X-rays are called fluoroscopy images. The practitioner can also acquire 3D images. During the acquisition of the images, the tube and the detector move about the patient, in a rotation of 40 degrees per second for example, in theory covering at least 194 degrees. Several projected images are then acquired by the C-type arm and, from these images, the part of the body to be viewed may be reconstructed in three dimensions. The contrast of these 3D images may be improved through an injection of a contrast agent.
The fluoroscopy images are acquired in real-time while the 3D image, which is generally a pre-operation image, is fixed. To improve the tool guidance system, there is a prior art method of projecting the 3D image on the fluoroscopy image. This method is called 3D enhanced fluoroscopy. It is also possible to project a 3D image on a pre-operation 2D image, for example a DSA image. There is also a prior art method whereby, in reverse, the 2D image is back-projected on the 3D image. This method is called the 3D roadmap method.
The difficulty of implementing these methods lies in the merging of the two images in taking account of the right acquisition geometry of the system. When projecting the 3D image on the 2D image or vice versa, it should be possible to determine the acquisition geometry of the imaging system whatever the position of this system in space. The acquisition geometry of the system is relative to the positioning of the tube and detector in a given reference system. This acquisition geometry is defined by the spatial position of both the C-type arm and the table, relative to a given referential system. The degree of freedom of the table can be modeled without great difficulty, by the use of prior art models. The C-type arm is more difficult to model.
A considerable effort is made to compute projection matrices that make a point located in the 2D image correspond to a point located in the reconstructed 3D image. A pixel of the 2D image is supposed to correspond to the projection of a 3D voxel of the reconstructed 3D image on the X-ray detector, in as much as this image will have been placed on the body. It should be possible to produce one projection matrix for each position of the C-type arm in space. This projection matrix is associated with the acquisition geometry of the system.
The method described in WO03/084380 proposes to integrate information produced by sensors of the vascular C-type arm into a rigid model of the C-type arm, in order to produce the projection matrices. The thesis by Erwan Kerrien, “Outils d'imagerie multimodalité pour la neuroradiologie interventionnelle” (Multi-Modality imaging tools for interventional neuroradiology), describes a method in which the projection matrices are computed from a calibration of positions of the C-type arm and the computation of a certain number of geometry parameters. At the SPIE Medical Imaging 98 conference in San Diego, USA, February 1998, Erwan Kerrien et al. in “Machine precision assessment for 3D/2D digital subtracted angiography images registration”, proposed a method pertaining to subtractive angiography. FR-2848806, describes a method for the calibration of a radiology imaging apparatus requiring a limited number of acquisitions. This method is based on a linear interpolation of matrix parameters of calibration matrices. This calibration method works only for the calibration of an axis of the C-arm, with the aim of a 3D image reconstruction. In “Optical configuration for dynamic calibration of projection geometry of X-ray C-arm systems”, Nassir Navab describes a method in which a CCD camera is attached to the X-ray detector. In this method, the geometry of the camera is used to compute the projection matrix. It is a method of measurement and not a method for the prediction of the projective matrix. In “Modeling the acquisition geometry of a C-arm angiography system for 3D reconstruction”, Cristina Canero et al. model the C-type arm in considering the intrinsic parameters of the projection matrix to be constant.
However, these prior art methods have limits as regards the modeling of the C-type arm. For the results of the projection of a point in space on the 2D image are not sufficiently precise. These methods are generally based on a rigid model of the C-type arm with constant internal parameters. This rigid model assumes the existence of unique axes of rotation about which the C-type arm is likely to rotate. However as a result of mechanical distortion undergone by the C-type arm and play between certain parts of the medical system, this rigid model, which may be called an ideal model, is often put at fault and the results obtained are not sufficiently precise for medical applications such as angiography. In particular, the theoretical nature of the trajectory of the ends of the C-type arm does not take account of the (great) weight of the X-ray tube and/or of the detector that causes this C-type arm to sag in proportions that are always variable.